This invention relates generally to booklet thickness calipers used on multiple feed head collating conveyor sysrems. More particularly, it relates to such calipers that provide electronic signals, compatible with electronic printing devices used with present collating conveyor systems and indicating whether a booklet engaged in said calipers is of proper thickness and has been properly collated.
Previous collating conveyor systems used a mechanical arrangement of parts extending from a pair of caliper rollers, one fixed add one movable, to set a switch if an undersigned or oversized collated booklet passed between the rollers while moving along a conveyor belt. The set switch disabled the binding stapler and effected a rejection of the booklet. More recently, the thickness of the booklet has been measured by reading analog electrical signals indicating the position of the movable roller. See, for example, U.S. Pat. No. 4,121,818 to Riley et al. A general purpose digital computer controller having an analog to digital converter uses the digitized position signals to determine the thickness of the booklet and ascertain whether it has been properly collated based upon preliminary measurements obtained through the calipers and compensation factors derived from the measurements during a make-ready or trial run mode. In the Riley et al. system, these preliminary data base determination measurements and compensation factors are used to compensate for booklet thickness variations resulting from various factors.
In the initial set up or trial run mode, the Riley et al. system takes caliper measurements of (a) three base or zero readings, with no signatures in the calipers, (b) three sample signatures from each signature feeder, (c) three standard replacement books, and (d) three books formed from all available signatures. The three values from each measurement are averaged and the average base reading is subtracted from each other average. These results, representing the average thickness for each signature, a standard book and a book containing all signatures, are stored, and an analysis of these data is performed.
The Riley et al. system then derives from the measured data reference an "air factor", which is the thickness of the air expected to be trapped between the signatures as they pass through the calipers. This is obtained by subtracting from the measured trial run thickness of the book containing all the sample signatures, the sum of the measured thicknesses of all the sample signatures, and dividing the result by the number of air interfaces. The air factor then is added to the measured thickness of each signature to result in a compensated thickness for each signature that is stored in a look-up table.
The Riley et al. system then stores a "thick tolerance" equal to the smallest compensated signature thickness and stores a thin tolerance equal to half that smallest compensated signature thickness. Lastly, the system computes and stores a "floating factor" determined by subtracting from the average measured thickness of the sample standard replacement booklets the sum of all the compensated thicknesses of all the signatures comprising the same.
In the production mode, the system adds together the compensated thicknesses of all the signatures to be incorporated in the booklet being produced during production and subtracts that sum from the actual reading obtained from the calipers for that booklet when it is completed. The floating factor then is added to the result of this subtraction operation and this second sum is compared to the "thick" and "thin" tolerances. If the difference plus the floating factor is outside the "thick" and "thin" tolerances, the book in the calipers is rejected as over- or under-sized. If the difference plus the floating factor is within the "thick" and "thin" tolerances, the book is of proper size and passes through the collating system. In this later case, the difference between the actual reading from the caliper and the calculated expected reading is added to a summer circuit that initially included the value of the floating factor. After seven additions to the summer, the new sum is averaged and is used as the new floating factor to compensate for slowly changing conditions.
There is at least one progressively varying condition however, that often occurs during a production run of booklets that may not be taken into account by the booklet production process of the Riley et al. patent, or if taken into account, is not handled in a simple or straightforward way as in the present invention. The ink from the booklets, especially if there is heavy coverage, can substantially build up on the reference roller and on the movable or measuring roller to effect a gradual change in the zero position of the calipers. This build up is gradual and evidences itself most at the end of a production run. It is non-existant at the beginning of a new run because operators clean the caliper rollers between production runs as a routine operation. The effect of the build up on the calipers is to slowly and gradually increase the apparent measured thickness of booklets engaged between the calipers so that the booklets are measured to be thicker than they actually are. This can result in properly collated booklets being rejected as oversized.
The thickness of the ink on the caliper rollers at the end of a production run typically can be one to two thousandths of an inch in a worst case condition. Previously, this had not been a problem because early mechanical systems were set to tolerances looser than the error introduced by the ink build up. The measurement made by more recent electronic caliper devices, such as is disclosed in the Riley et al. patent, however, can appreciably vary with this ink build up and, in such cases, can affect the control computer's decision as to whether or not an improperly collated booklet is being measured by the caliper. While the ink build up error can be overcome in electronic caliper devices by expanding the tolerances for properly collated booklets, such as was done in prior mechanical systems, this is unsatisfactory because it defeats the purpose of using fine electronic measuring systems. In electronic caliper systems, some means should be provided to account for this ink build up or zero offset because ultimately it can prevent an accurate precise measurement of the thicknesses of various booklets engaged between the caliper rollers.
Additionally, the caliper device used in the structure of the Riley et al. patent is cumbersome and requires that several heavy parts be moved in translation to sense the thickness of a booklet. See the Abram et al. patent, U.S. Pat. No. 3,899,165, FIG. 4 for a picture of the structure used in both the Riley et al. and Abram et al. structures. In particular, that structure includes a linear variable-differential transformer having a probe shaft that is longitudinally moved in translation through stationary transformer coils. The probe comprises the shaft moving through the coils, a roller fixed to one and of the shaft, and a measuring disc that engages with the roller, the measuring disc and its shaft being moved in turn by a booklet passing between it and a reference cam. All of these parts must be moved in translation to effect a measurement. Further, the disclosed structure is so massive that a dash pot is desirable to absorb the inertia of the moving probe.
With the advent of fine measuring systems providing precise measurements, heavy mechanical parts that must be moved in translation to effect a thickness measurement of a booklet are undesirable. They have too much inertia and a simple, lightweight device is desired that has a minimum inertia.